Personal Communications Services ("PCS") are the focus of an exceptional amount of interest, both in the United States and around the world. The global telecommunications network today forms the infrastructure for our information-based society. The need for instantaneous communications capability is critically important in today's information economy, as the unprecedented growth of cellular telecommunications demonstrates. PCS will permit millions of people worldwide to initiate person-to-person communications, using small and inexpensive low-power telephone handsets. People no longer will be tethered to stations at which telephone calls may be placed and received, but will be able to communicate freely whether at home, at work, or in the public environment.
PCS will enhance the nation's productivity and the quality of our lives. It will be a truly personal service. Although PCS will provide competition to cellular, paging, and other services, it will be unlike cellular--at least 95 percent of the airtime of which is paid for by businesses--in that PCS will make portable communications available to great numbers of people who have been unable to participate in today's mobile telecommunications revolution. It will enable Eastern Europe and Third World countries to leapfrog over their crippled infrastructure handicaps. It will contribute formidably to American exports and economic well-being. In the long term PCS may eventually provide competition to the telephone companies' local exchange monopolies.
PCS systems are comprised of multiple base stations that may be linked in a variety of ways to comprise an integrated telecommunications system. Base stations receive communications from PCS handsets and route those communications to the intended source, using either the PCS system independently or using the PCS system to route the communications to the public switched telephone network ("PSTN"). Base stations also transmit communications to PCS handsets from other sources on the PCS system or from the PSTN. PCS systems may incorporate independent telephone switching and control centers or PCS operators may cooperate with local exchange carriers to utilize existing switching facilities operated by those carriers. Subscriber access to the PCS system is established by a radio communication link between the PCS handset and the base station.
The geographic area served by PCS operators is divided into a number of zones or cells, each of which is served by at least one base station. Dividing the geographic area into relatively small cells permits extremely efficient use of the spectrum because frequencies can be reused in geographically separated cells. Frequency reuse also permits large numbers of subscribers to be se system because numerous frequencies may be available in a given cell. PCS systems also may incorporate the capability of transferring a subscriber unit with a call in progress from cell to cell as the subscriber moves within the service area.
The implementation of PCS technology differs from traditional cellular telecommunications in that cellular systems have been designed with cells of comparatively large radii, requiring relatively high-power subscriber units. This design follows logically from the basic initial purpose of cellular telephone--to permit mobile telephone use in automobiles. Although hand-held portable cellular units have been developed, the comparatively high power required by cellular systems limits the ability of such handsets to use small and light power sources. PCS systems, in contrast, use a great number of much smaller radius cells, permitting PCS handsets to operate at much lower power because the handset-to-base station signal need only travel a relatively short distance. PCS handsets can thus be small, light and inexpensive. PCS handsets also can have applications in private local-area networks within buildings and for data transmission.
The essential distinguishing technical characteristic of PCS is that the frequencies identified for PCS by the U.S. Federal Communications Commission and spectrum-allocation bodies throughout the world are currently occupied by other users. In the United States and several other countries, PCS will be required to share this spectrum with existing users. The FCC has indicated that it is likely to allocate the 1.85-1.99 GHz band to PCS. This band is occupied by private operational fixed microwave users, which use the band for point-to-point microwave transmissions. The challenge to PCS operators will be configuring systems around existing users without causing interference to those users. As a comparison, cellular licensees have clear spectrum and thus are concerned only with intrasystem (as opposed to intersystem) interference. U.S. Pat. Nos. 4,144,412, 4,736,453, 4,481,670 and 4,485,486 are examples of such concepts, which will not account for interference concerns between PCS licensees and point-to-point microwave users.
Point-to-point microwave systems can employ different power levels, antenna gain, modulations, channel bandwidths, and other technical characteristics, and microwave usage varies significantly from area to area. In some areas, up to 100 MHz of spectrum with the 1.85-1.99 GHz band may be available for implementation of PCS, while in other areas as little as 25 MHz of spectrum may be available. To meet this challenge, the PCS industry will need a system and method for engineering PCS frequency use around existing microwave users that can be readily adapted to any area and most efficiently make use of the available communications spectrum in that area. Such a system must fully protect existing microwave users from interference and should take advantage of PCS technology advances in equipment and ensure that these advances are not inhibited nor hampered. The invention described in this Application is designed to serve this need.